Constant pressure units are generally referred to as constant pressure dampers or grilles and are used in the control of the airflow in and out of pressured "clean" environments such as hospital operating theatres, electronic components fabrication, and all other applications requiring a tight control on contaminants entering the "clean conditions" environment. As background information to this invention, reference is made to British Patent No. 931,526 published Jul. 17, 1963 and entitled "Improvements in and Relating to Damper Valves".
In the application of the constant air pressure unit as a terminal unit in air handling systems, the unit is used to divert excess conditioned air entering the unit as the airflow leaving the unit is varied. This is generally referred to in the art as a by-pass terminal unit. As background information, reference is made to U.S. Pat. No. 5,044,402 relating to a variable air volume terminal unit as well as to our copending U.S. application Ser. No. 08/165,603, filed on Dec. 13, 1993 relating to a sliding gate terminal unit for an air handling system. The underlying principle in air handling systems using by-pass terminal units is that the airflow at the supply fan remain constant and, in consequence, the airflow at each terminal unit should remain constant as the conditioned air delivered to each space to be conditioned varies as the requirements of the space being conditioned change- With by-pass terminal units of the prior art, the airflow at the inlet increases as the unit cycles the airflow between the outlet opening and the by-pass opening, the maximum increase being attained at or near mid-cycle. To understand this problem with the prior art, let us consider the flow characteristics of the air handling duct system. The individual duct systems upstream and downstream from the by-pass terminal unit have their predetermined airflows and thus their predetermined pressure loses since they are of fixed cross-sections and length. The quantity of pressure required to move the desired amount of air through the ducts varies as per the following formula: EQU Pressure=Constant x Velocity.sup.2
where the constant depends on the duct geometry.
To reduce the quantity of air in a given duct system while maintaining the same pressure at the entrance, it is necessary to increase the resistance to the flow of air in the duct system, i.e. add restrictions such as dampers. For example, let us define three duct systems: one from the supply fan to the inlet of the by-pass box, one from the inlet of the by-pass box to the conditioned space and one from the inlet of the by-pass box through the by-pass outlet and a manually adjustable balancing damper. Let us suppose for case of argument that through the use of the manual balancing damper, the frictional losses are equal in the two later duct systems in this example. In by-pass units of the prior art, as the diverter blade moves the conditioned air from one of these two later duct systems to the other, the velocity in one duct will drop as the other will increase from zero. In the mid-cycle position of the diverter blade, the air velocity in each duct system will be half of its previous maximum: there are now two outlets for the same quantity of air. With the reduction in air velocity, the frictional losses have been reduced and less pressure is now required to deliver the air to the conditioned space and the by-pass outlet. In the prior art, several shapes and methods of mounting the diverter blade have been proposed without adequately compensating for this problem. In response to the change in pressure requirements, the velocity through all three duct systems will then increase until the loss of pressure has been compensated for. This translates into an increase of the airflow at the supply fan. In the case where several by-pass boxes in the system are subjected to the same conditions, the increase in airflow at the supply fan may become more than the supply fan motor can handle. In known by-pass boxes on the market today, supply fan motors must be oversized to protect against the above eventuality and use more power than would be theoretically necessary if no variations in system pressure were created by the by-pass box.